I built this motorcycle voltage regulator
**broken link removed**
quoting from the designer:
"Capacitor C1 stabilises the circuit at higher load. I have observed that the switching behaviour of the circuit without this capacitor is pretty "sharp" and stable if the load draws less than ca. 0.5 A (typically); however at higher current I observed a tendency to oscillate. This can be observed best with an oscilloscope, or a 12 V lamp as load: it will start to glow, instead of being switched completely on or completely off. C1 eliminates this instability; a value of 100 nF to 220 nF should do the job."
It does indeed oscillate, as I found, even using a mosfet instead of the darlington.
The designer's solution of putting a cap on T1 slows down the switching, so I'd like to find a better way. I guess the field switch doesn't cause the problem, because it happens whether you use a darlington or a mosfet. That points to T1. Why it would not oscillate with a small field current and then start oscillating at higher field current I'd like to understand.
Perhaps a base resistor or a tiny bit of inductance would help?
**broken link removed**
quoting from the designer:
"Capacitor C1 stabilises the circuit at higher load. I have observed that the switching behaviour of the circuit without this capacitor is pretty "sharp" and stable if the load draws less than ca. 0.5 A (typically); however at higher current I observed a tendency to oscillate. This can be observed best with an oscilloscope, or a 12 V lamp as load: it will start to glow, instead of being switched completely on or completely off. C1 eliminates this instability; a value of 100 nF to 220 nF should do the job."
It does indeed oscillate, as I found, even using a mosfet instead of the darlington.
The designer's solution of putting a cap on T1 slows down the switching, so I'd like to find a better way. I guess the field switch doesn't cause the problem, because it happens whether you use a darlington or a mosfet. That points to T1. Why it would not oscillate with a small field current and then start oscillating at higher field current I'd like to understand.
Perhaps a base resistor or a tiny bit of inductance would help?
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